depending on what language you already have experience in, you will take a test either before you in-process or shortly after you arrive to test your proficiency in that language. Depending on how well you do, you might be placed in that language at a higher lever class.
I never took the DLAB during basic, I don't know why they didn't do it for 2014. they had us rate our choice of language from 1 to 3. Luck fully I got my first pick (arabic). however, I know a lot of people that got stuck in something they didn't want. So depending on if they do it the same for 2015, it really comes down to luck and if you have experience in a certain language.
However, if you do get assigned a language you don't want, its possible for you to switch. especially if you want to be place in a strategic language. that being said, it's sometimes really hard to convince the registrar to change your language.